1. Field of the Invention:
The present invention is generally directed to solar cells and, more particularly, to improved metallized grid patterns for such cells.
2. Description of the Prior Art:
The use of solar cells to convert solar light into an electrical current is well known. The cell current is directly proportional to incoming solar light intensity, hereafter referred to as solar intensity and the area of the exposed cell surface. One type of a solar cell which has been used and to which the present invention is directed comprises a slab of semiconducting material which is doped to either the "N" or "P" polarity, so that the majority carriers are either electrons or "holes" respectively. The semiconducting material is generally referred to as the cell base.
On the cell base is a very shallow, e.g., 3,000A or less, counter-doped layer, whose outer surface, which is exposable to solar intensity represents the top cell face. The counter-doped layer is doped to a "P" polarity when the base is an "N" base and to an "N" polarity when the base is doped to a "P" polarity. The cell also includes a bottom electrode, generally in the form of an electrically conductive metallized layer deposited over the bottom side of the base, and a top electrode which is deposited on the top of the counter-doped layer, i.e., on the top cell face. Solar photons passing through the counter-doped layer into the cell base generate electron-hole pairs. The doped-counter doped regions result in creations of an internal electrical field so that with a "P" doped base and an "N" doped counter-doped layer photogenerated electrons are injected from the base into the counter doped layer where they become majority carriers travelling to the top electrode, while the photogenerator holes as majority carriers travel to the bottom electrode. Similarly, with an N base and a P counter-doped layer holes are injected into the counter-doped layer and electrons travel to the bottom electrode. The counter-doped layer presents a very small cross sectional area to the injected majority carriers which must eventually reach the top electrode, and additionally, the resistivity of the counter-doped layer is quite high in comparison with the resistivity of metals such as silver or aluminum. Consequently, in order to reduce this considerable electrical resistance, presented to the majority carriers flowing through the counter-doped layer to the top electrode, an electrically conductive metallized grid pattern is deposited on the active cell surface, i.e., on the top cell face, which is exposed to the solar intensity. The grid pattern is in electrical contact with the top electrode so that once majority carriers reach any part of the grid pattern, the resistance presented to them is that of the metallized pattern.
As used herein, the term solar cell intends to refer to a cell with a cell base doped to one polarity and a counter doped layer doped to another polarity. The invention is directed to the grid pattern so as to reduce the series resistance presented to the majority carriers in the counter-doped layer.
In many known prior art solar cells, grid patterns are employed in which the path length which majority carriers have to travel to a conductive grid of the pattern increases dramatically with distance from the cell's geometric center. Also, in others, some of the path lengths are unnecessarily long thereby producing high cell series resistance. These characteristics are most disadvantageous, particularly at high solar intensities, which are present near the sun or when solar concentrators are employed. In the prior art cell series resistance is high and therefore power dissipation in the cell in the form of heat is high, thereby decreasing the cell's efficiency. Furthermore, in the prior art due to the grid patterns employed therein, large temperature differentials across the cell's face are experienced, which is particularly undesirable at high solar intensities. Large temperature differentials lower the point at which cell power begins to decrease with increasing solar intensity.